Freely definable variable

I need a freely definable variable for my trigger.
e.g. #{config.custom.foo}

This variable should be able to be set with an environment variable. It is not necessary in the web frontend.

Since I control all triggers from my CICD, I want to be able to adapt trigger texts to the respective environment (testing, staging, production) programmatically.

this is a trigger text on the instance #{config.custom.instance} with the URL #{config.fqdn}!

should be set (when config.custom.instance=production)

this is a trigger text on the instance production zammad.example.com!

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